Modern communications systems use forward error correction (FEC) coding in order to achieve extremely low bit error rates. In single hop systems (e.g., the transmission of data from a transmitting terminal directly to a receiving terminal), an FEC encoder in the transmitting terminal encodes an input stream of user data at a given data rate (i.e. the user data rate) to produce a stream of encoded data, which comprises extra bits. The FEC encoder outputs the stream of encoded data at a higher data rate (i.e. the encoded data rate). The ratio of the user data rate to the encoded data rate is called the code rate. This stream of encoded data is then modulated in the transmitting terminal, and transmitted to the receiving terminal, where it is demodulated. An FEC decoder in the receiving terminal then uses the extra bits generated by the FEC encoder to correct for errors introduced by the communications channel.
Satellite communications systems also employ FEC coding to achieve low bit error rates, but the fact that there are two hops (e.g., the transmission of data from a transmitting terminal to a satellite, and then from the satellite to a receiving terminal) increases the number of architectural options. Each option involves different arrangements of modulators, demodulators, FEC encoders, and FEC decoders. Better performance can be achieved by performing more processing onboard the satellite at the expense of increased cost and complexity. Looking at the options studied to date, this tradeoff favors simpler satellites because the level of complexity needed to achieve significantly better performance has been cost prohibitive.
There is therefore a need for an improved satellite communications system architecture that provides better performance than a simple transponder architecture without adding significant cost and complexity to the satellite.